Why same scene has diferent size when i export it to three.js and babylon.js
There are many reasons this could happen:


three.js and babylon.js formats are just different, and may encode things in different ways
the three.js exporter may be including extra data that isn't in the babylonjs file like normals or tangents (whether that's data you want or not, who knows)


tl;dr this is fine and normal. Whichever engine you decide to use, both support many formats (FBX, glTF, OBJ) and some may be more efficient than either JSON format. glTF and Draco are may give smaller sizes, in particular.